DETERMINATION OF THE SHEAR STRENGTH PARAMETERS ASSOCIATED WITH MUDFLOWS

抄録

When a mudflow moves down a slope, the gravity induced shear stress, τ, is opposed by the combined effect of the undrained shear strength, c_u, of the mud, and its viscosity, η, which is associated with the velocity gradient in the mudflow, dv/dy. In equation form this can be expressed as : τ= c_u+η(dv/dy). The resistance provided by the mud involves the contribution of both c_u and η. For an understanding of the mobilization mechanics of mudflows, one needs then these two parameters. This study describes two methods that can be used to determine these parameters. The c_u of the muds is measured using a cylinder-strength meter devise. The technique associated with this devise consists in lowering a cylinder of known dimension and weight into a mud sample, measuring the depth of penetration of the cylinder into the mud, and calculating the strength, c_u, of the mud required to support the solid cylinder at that depth. The c_u of the mud is calculated using Sokolovski's theory designed to calculate the indentation pressures developed by a Tresca plastic when a cylinder penetrates it. The determination of the viscosity, η, of the mud is obtained from either the velocity measurements of the free surface of the mudflow, or from the difference between the velocity of the free surface of the mudflow and its velocity at the interface between the mudflow and the surface on which it moves. The viscosity η is obtained from a relationship that relates these velocities with η.